Understanding Model Based Definition

Model Based Definition (MBD) is an annotated 3-D CAD model that contains all information that is needed to define a product.

Annotations are dimensions, notes, symbols, surface finishes, and other text or markings. Used to convey engineering intent.
The annotated model replaces a traditional drawing. All annotations are created in the 3-D model.
The model is now the master.
The drawing is now optional.
ASME 14.41 is used to define the presentation of annotations.
MIL-STD-31000A Specification. Presents an organization schema for the data.
Why MBD? Engineering design efficiency.
Reduced time to market.
Higher quality products through increased understanding.
Model Based Enterprise (MBE) Making MBD models and all related information available to the engineer and enterprise.

Figure 1 - Viewing a 2-D Drawing

Figure 2 - Viewing an MBD Model

Understanding Model Based Definition

Figure 1 - Viewing a 2-D Drawing

To understand model based definition (MBD), it is necessary to take a step back and look at the history of drawings. Drawings were created to standardize hand drawn sketches. The drawing was the master definition of the product. This process of creating a drawing was enhanced with the advent of Computer Aided Design, or CAD.

Not long after 2-D electronic sketch pads were created, it was realized that 2-D drawings could be created faster and more accurately by projecting views from a 3-D CAD model. All the geometric information was contained in the 3-D model. Additional information, such as notes, dimensions, symbols, and other tolerancing details, was put in the drawing.

The Rise of MBD

Figure 2 - Viewing an MBD Model

However, 3-D visualization provides a richer, more complete environment for users. A turbine blade, for example, is hard to perceive with a 2-D drawing. But view a 3-D model of the turbine blade and it is very easy to understand it. Consequently, MBD was created. MBD is an annotated 3-D CAD model that contains all information needed to define a product. The annotated model replaces a traditional drawing, and thus the 3-D model is the master.

The formal definition of MBD is as follows:

MBD is a set of concepts, processes, and tools that enable the creation of an annotated 3-D product definition based on a 3-D solid model. The MBD dataset includes all Engineering Intent (including Process Specifications, Geometric Dimensioning and Tolerancing (GD&T), and other required information). Combined with product lifecycle management (PLM) attributes, a parts list, and general notes, this constitutes an authoritative, single source of master product definition data that does not include or depend upon traditional 2-D drawings. The MBD dataset defines complete requirements for a product in its nominal condition as well as permissible limits of variation and other acceptance criteria, providing all the data needed to plan, fabricate, and validate an article of production hardware.

Specifications Creation

To make the 3-D model the master and remove the criterion of creating the derivative 2-D drawing, it is necessary to take all the items that must be shown on a drawing and place them into the 3-D model, including symbols, datum tags, surface finishes, notes, and so on. These items are typically on multiple drawing sheets, each containing multiple views. However, if all this information is placed into the 3-D model it may become unreadable due to the amount of information that needs to be displayed and related to a relatively small geometric area.

Organization is key to the implementation. A schema for dividing up model information into commonly named groups was devised and incorporated into MIL-STD-31000A. These groups split the information based on usage or functional topics. With a standardized naming convention, specific information can be found consistently and quickly.

Why MBD?

MBD is not paperless engineering. It does not depend on passing data from one engineering, design, analysis, or process planning tool to another. It does depend on sharing technical data to the extended enterprise and communicating with the supply chain on manufacturing and purchasing decisions. The primary reason for adopting MBD is efficiency. The efficiency may not be seen in engineering, however, because instead of detailing a drawing, a model is annotated. But the downstream benefits are monumental in terms of time to market and the increased reuse of information.

Model Based Enterprise

MBD is part of a larger environment called Model Based Enterprise, or MBE. The formal definition of MBE is as follows:

MBE is a fully integrated and collaborative environment founded on 3-D model based product definition, constructed and shared across the enterprise to enable the deployment of products, from conception through end of life cycle.

In other words, MBE is a process of reusing the 3-D CAD model by all of downstream consumers versus recreating or re-entering the data it contains. This model contains all of the information needed to define the product in an organized manner. That information can also be easily accessed, parsed, viewed, or queried by electronic means or by users within the enterprise and throughout the supply chain.